A number of catalyst compositions containing single-site catalysts have been used to prepare polyolefins, producing relatively homogeneous copolymers. In contrast to traditional multi-site Ziegler-Natta catalyst compositions, single-site catalysts, such as metallocene catalysts, are catalytic compounds containing uniform polymerization sites.
To achieve acceptable and commercially viable polymerization activities with metallocene catalysts compounds, a large amount of activator such as methylaluminoxane (“MAO”), is often required. Unfortunately, such activators are expensive and the amount of activator required to produce an active catalyst for polymerization has been a substantial impediment to the commercialization of metallocene catalysts for polyolefin production. Therefore, there is a need for catalysts that have improved activity and productivity, and thus require less activator.
Mixed catalyst systems frequently include at least one metallocene catalyst compound to produce bimodal polyolefins. The production of bimodal polyolefins using a mixed catalyst system that includes at least one metallocene catalyst compound often require a large amount of activator and/or a large amount of the metallocene catalyst compound in order to produce commercial quantities of bimodal polyolefins. There is a need, therefore, for improved methods for producing bimodal polyolefins with mixed catalyst systems.